Cellulose nitrate is known as an essential constituent of many paint systems (cf. e.g. U. Biethan, Lacke und Losemittel [Paints and Solvents], Verlag Chemie GmbH, Weinheim, 1st edition, 1979, p. 8).
Since cellulose nitrate in the dry form can be made to ignite very easily by impact or friction, commercially available paint cellulose nitrates must be treated either with a stabilizing agent (e.g. humidifying agent) in an amount of at least 25 wt. % or a gelatinizing plasticizer in a concentration of at least 18 wt. % in order to reduce risks during storage, handling and transportation.
In addition to water, in general alcohols, such as ethanol, isopropanol and butanol, are used as humidifying agents. However, in the processing of paints containing cellulose nitrate, alcohols cause trouble in the conventional adsorption units installed downstream of painting units to reduce emissions. Alcohols also cause trouble when used in PU paints. They react with isocyanates to give low molecular weight compounds, as a result of which the quality of the paint film is reduced significantly.
DE-A 30 41 085 and U.S. Pat. No. 3,284,253 describe cellulose nitrates which contain inert solvents (e.g. toluene) as a humidifying agent. They are prepared by means of a distillative working step, which is exceptionally expensive and presents safety problems.
All cellulose nitrates provided with humidifying agents continue to have the problem of evaporation of the humidifying agent or inhomogeneous distribution as a result of sagging during storage in a drum.
So that the user has a free choice of solvent which can be employed, cellulose nitrate is also available in plasticized form (so-called NC chips). These NC chips are generally prepared by incorporating plasticizers into water-moist cellulose nitrates. The preparation is described e.g. in DE 1 203 652, DE 1 570 121, DE 1 470 860 and DE 2 338 852.
Prerequisites for the preparation of NC chips are gelatinizing plasticizers in a pure form or those plasticizers which acquire gelatinizing properties by an addition of organic solvents which dissolve cellulose nitrate. These organic solvents are removed again during drying. Environmental technology measures are therefore necessary in such a preparation process, in order to ensure the legally required emission values and explosion limits are adhered to.
In addition to the gelatinizing power of the plasticizers with respect to the cellulose nitrate, additional plasticizer properties such as acceptability in terms of foodstuffs legislation, resistance to migration and good general mechanical and chemical properties are required by the market as a basic prerequisite for the preparation of NC chips.
It is known that polyurethanes meet a large number of the properties required. They are distinguished e.g. by good resistance to chemicals, resistance to heat, fastness to light (in the case of polyurethanes based on aliphatic isocyanates) and very good mechanical properties.
The preparation of cellulose nitrates plasticized with polyurethane oils is described in DOS 2 215 624. According to the invention, an aqueous suspension of the plasticizing agent is brought together with an aqueous suspension of cellulose nitrate fibres with vigorous stirring. The resulting product is filtered off, squeezed off, shaped by calendering, dried and cut to chips. Anionic and/or nonionic emulsifiers are employed for the preparation of the suspension of the plasticizing agent, and these remain in the NC chips after the preparation process and impair in an undesirable manner the resistance to chemicals and in particular the resistance to water of coatings produced therefrom.
To avoid the disadvantages described, it would be desirable to be able to prepare such NC chips by direct application of polyurethane plasticizers on to the NC fibres. However, this has hitherto not been possible because of the lack of gelatinizing power of polyurethane plasticizers or polyurethane oils known to date.